Efficiency of transformer = (secondary coil ÷ primary coil) × 100%

Efficiency of transformer = (mechanical advantage ÷ velocity ratio) × 100%

Efficiency of a transformer = (Number of turns in the secondary coil ÷ Number of turns in the primary coil) × 100%

Efficiency of a transformer = (power developed in the secondary coil ÷ power supplied to the primary coil) × 100%